Am after tinting the windows on my motor but want to ask what's legal for rear side windows and rear back window

I've seen some advertised and they seem to come in
5%
15%
35%

you can have them as light or as dark as you want, rear windows theres no law , its fronts where you have to be careful

mine are pure black on the back, not done the fronts yet, the windows are tinted from factory so have to be careful not to overdo them, this should be taken into account when doing the fronts.

Need to do my rear window...

Thanks folks I'll get the stuff ordered today then and I'll post a pic when I've done it